1. Field of the Invention
The present invention relates to a coreless electric motor having an axial gap, being especially thin so as to be used such as a blower of a car radiator or a driving source of a copy machine and so on by being driven by a direct current power source.
2. Description of the Related Art
A core-less electric motor can be made flat by having an axial gap. Recently there is a demand for motors being thinner, lighter, lower in acoustic noise and lower in cost so as to be used as parts of cars or a copy machine.
First, a conventional coreless electric motor is explained with reference to the accompanying drawings.
FIG. 4 is a cross-sectional side view of a conventional coreless electric motor. As shown in FIG. 5, a doughnut shaped field magnet 1 is alternately magnetized N and S in a rotational direction, and it is fixed on the inside wall of an outer frame bracket 2 by using an adhesive, or the like. A coreless armature 3 has a flat disk shape and is disposed against the field magnet 1 with a little gap inbetween. An inner frame bracket 4 which also serves as a magnetic yoke is disposed against the armature 3 with a small gap in between. A brush 5 which is for feeding an electric current for the armature 3 is fixed on the inner frame bracket 4. A shaft 6 which is connected with the armature 3 is held by a bearing cover 8 through bearings 7A and 7B.
FIG. 5 is a front view of the field magnet 1.
As shown in FIG. 5, the field magnet 1 has radial non-magnetized parts 9a, 9b, 9c, 9d, 9e, 9f and 9g between the magnetic poles. Magnetic flux flows in a path which is from an N-pole of the field magnet 1, through the gap 10A, the armature 3, gap of the inner frame bracket 4, gap 10B, the armature 3, gap 10C, an S-pole of the field magnet 1, the outer frame bracket 2 and to the N-pole of the field magnet 1 in FIG. 4. At this time by being fed with an electric current through the brush 5, the armature 3 begins to rotate in accordance with Fleming's rule. As above-mentioned, the conventional coreless electric motor has an axial gap for reducing its thickness. It is, however, insufficient to reduce the thickness because the field magnet 1 and the brush 5 are disposed together in the axial direction.
FIG. 6 is a cross-sectional side view of another conventional coreless electric motor.
As shown in FIG. 6, the core-less electric motor has a brush 5 and a field magnet 1 disposed in a concentric relation in regard to the axis of an armature 3. The motor is thinner, and an inner frame bracket 10, inner frame bracket 4 and bearing cover 8 are integrated in one body as shown in FIG. 4 to reduce its weight. In this case, the flow of the magnetic flux uses the outer frame bracket 11 and the inner frame bracket 10 as the magnetic yoke similarly to the coreless electric motor in FIG. 4. In such case, the reaction of the armature 3 caused by feeding an electric current thereto makes the magnetic flux of the field magnet 1 nonuniform. Thereby, when looking at a point on the surface the outer frame bracket 11 which is disposed against the field magnet 1, periodic nonuniformity of the intensity of the magnetic field with respect to time results. Thus, strain which is determined by rigidity of the outer frame bracket 11 induces periodic nonuniformity as time lapses, so that acoustic noise due to magnetic oscillation is produced from the outer frame bracket 11.
In the conventional coreless electric motor, to reduce the acoustic noise and the stress, the thickness of the frame has been changed, but it naturally has resulted in a problem of heavy weight for the apparatus.